Polycomb group (PcG) proteins are chromatin modifying enzymes that are dysregulated in many human cancers. The Polycomb Repressive Complex 2 (PRC2), which includes SUZ12 (suppressor of zeste 12), EED (embryonic ectoderm development) and the catalytic subunit, EZH2 (enhancer of zeste homolog 2), represses genes by methylating the core histone H3 lysine 27 (H3K27me3) at and around the promoter regions of target genes. PRC2 is the critical component of cellular machinery involved in the epigenetic regulation of gene transcription and plays critical function in development and tissue differentiation and regeneration. Although EZH2 is the catalytic subunit, PRC2 requires at least EED and SUZ12 for its methyltransferase activity. EED, SUZ12 and EZH2 are overexpressed in many cancers, including but not limited to breast cancer, prostate cancer, hepatocellular carcinoma and etc. EZH2 activating mutations have been identified in DLBCL (diffuse large B cell lymphoma) patients and FL (follicular lymphoma) patients. Inhibition of PRC2 methyltransferase activity by compounds competing with the cofactor S-adenosyl methionine (SAM) in DLBCL reverses H3K27 methylation, re-activates expression of target genes and inhibits tumor growth/proliferation. Therefore, PRC2 provides a pharmacological target for DLBCL and other cancers. In particular, the need exists for small molecules that inhibit the activity of PRC2. The present invention fulfills this need.